The M96 and M97 engines just have very unreliable components and poor design in many ways. The rest of the car as far as I can tell is reasonably reliable, maybe with the exception of the soft top mechanism, and "user error" like people leaving the window down and the ECU getting soaked by rain.
It's not difficult to find the problems with the cars. You can go to LN Engineering for example and read up on it. The M96 and M97 engines have an open deck design with relatively thin cylinder walls and are cast high silicon alloy (weaker than normal aluminum). They can outright have a chunk break off, but more commonly the bores eventually get oval enough for the oil consumption to make an early 1ZZ blush. There are not a lot of these engines that survive into the high 100k mile range.
The water pumps are known for failure and are recommended to be replaced every few years. The chain guides on 5-chain engines wore quickly, I don't know if they ever fixed that problem in the later engines. Spark plugs are much more involved than your usual engine since they're not accessible from the top (Subarus also have this issue though). Oil starvation, AOS failure, and rod bearing failure are common on tracked cars, because they put scavenge pumps at opposite corners in the heads, so I think left turns with a forward negative acceleration component will pool oil where there is no scavenge pump. The AOS is just bad, you have to pony up 4 digits for the "Porsche Motorsports" upgrade, or figure out how to tuck an aftermarket catch can or AOS in as a custom job. Apparently, hydraulic lash adjuster failure is somewhat common. The crankshafts are not very strong and will snap without a dual mass flywheel or damped crank pulley.
A rebuild is going to run you at least 10k.
Pretty much all those problems were solved on the later cars, so you don't need to budget huge amounts of money for repairs generally speaking. That's why 987.2s get snapped up so quickly when they hit the market. On a 987.2, compared to a MR2 Spyder you're just looking at more expensive tires, brakes, and oil (10 quarts!), and getting kind of shafted when you eventually do less frequent service items like coolant, transmission fluid, spark plugs, etc. since the Porsche fluids are special and expensive. I think if you do the easier work yourself or have a good priced mechanic, it's easy to stay under 1k/year in service cost. The parts are more money than most cars, but mostly not outrageous because there are a lot of Porsches and they share parts.
Once you move to the 981 and up generations, I think it gets a little more expensive because there is a bunch of computer stuff that you're supposed to have a dealer (or indie with the equipment) set when they do mundane things like batteries. Or, you could just ignore the error messages that pop up on the screen to save money lol.
